The invention relates to a method for patterning fine features for semiconductor devices using a phase shifting mask with no blocking material separating the opposite phases.
Patterned features making up integrated circuits are conventionally formed using photolithography and etch techniques. A photomask, which transmits light in some areas and blocks it in others, is formed, the blocking areas corresponding to the pattern to be formed on the wafer surface (or its inverse.) The surface to be patterned, for example a semiconductor or dielectric layer, is covered with a layer of photoresist, a photoreactive material. Light is projected onto the photoresist surface using the photomask, selectively exposing areas of photoresist. The wafer is then subjected to a developing process, in which exposed photoresist (or unexposed photoresist, in the case of negative photoresist) is removed, leaving patterned photoresist behind.
The remaining patterned photoresist then typically serves to protect underlying material during a subsequent etch process, creating features in the same pattern as the remaining photoresist.
Over the years integrated circuits have become denser and patterned features smaller. As projected features become smaller, the limits of resolution are reached and it becomes more difficult to project patterns with sharp edges. Poor resolution can lead to incomplete patterning and to incomplete etching or overetching, causing device flaws.
Alternating phase shifters, which invert the phase of light in some areas of the photomask, increasing contrast in light intensity at the photoresist surface, are a powerful tool to improve resolution and sharpen edges.
The use of alternating phase shifters in photomasks, however, has disadvantages. When alternating phase shifters are used, projected light is either incident, in what will be called zero degree phase, or inverted, in what will be called 180 degree phase (this is sometimes also called π phase.) As will be more fully described, as conventionally used, light in opposite phases is transmitted on opposite sides of an obscured area. The configuration of some patterns leads to phase conflicts, in which rules dictate that the same area must see light of opposite phases. To date, this has meant that use of alternating phase shifters has been limited to only certain types of patterns.
There is a need, therefore, for increased flexibility of phase shifting photomasks.